1. Field of the Invention
The present invention relates to a method of evaluating the quality of a silicon wafer, and a method of reclaiming the silicon wafer. By using the methods of the present invention, crystal defects present in the silicon wafer can be precisely be detected in a nondestructive manner. Therefore, the present invention is preferably used for evaluating the quality of a silicon wafer (a used silicon wafer) used in a process for manufacturing semiconductor devices, and reclaiming the silicon wafer.
2. Description of the Related Art
In the process for manufacturing semiconductor devices, a silicon wafer used as a test wafer or a monitor wafer is reused through polishing and cleaning after removal of various films (oxide films, and the like) formed on the surface thereof. Similarly, a silicon wafer for manufacturing devices, which becomes defective due to some abnormality in the process for manufacturing semiconductor devices is also reused as a test wafer or monitor wafer through film removal, polishing and cleaning. In the present invention, such a silicon wafer used for some purpose in the process for manufacturing devices is referred to as a xe2x80x9cused silicon waferxe2x80x9d. Among used silicon wafers, a silicon wafer reclaimed by any of various methods is referred to as a xe2x80x9creclaimed waferxe2x80x9d.
The practical use of the reclaimed wafer has the great advantages of reducing the manufacturing cost of devices, and permitting effective use of resources. However, even the reclaimed wafer is required to have the same quality level as a new wafer with respect to surface defects, fine particle adhesion, surface metal concentration, etc. The quality of the reclaimed wafer is generally evaluated at the time the reclaiming process is completely finished, and only the reclaimed wafer whose quality agrees with a predetermined standard is reused. Therefore, various reclaiming methods have been proposed for providing high-quality reclaimed wafers at low cost.
The method of reclaiming the used silicon wafer generally comprises the step of removing films formed on the used silicon wafer, the step of polishing the wafer from which the films are removed, the step of cleaning the polished wafer, and the step of evaluating the quality of the final product. Particularly, various methods of providing excellent reclaimed wafers have been studied, in which the film removing step is improved.
In the film removing step, lapping, grinding, chemical etching, chemical-mechanical polishing, and the like are performed singly or in combination. In another method, the types of films formed on the wafer are determined by visual observation, surface resistance, infrared absorption spectra, light reflectance, or the like, and then the films are removed according to the characteristics of the films. For example, Japanese Unexamined Patent Publication No. 9-17833 discloses a method of reclaiming a used wafer in which the types of films formed on a wafer are estimated by infrared absorption spectra, and grouped into several patterns, and then reclaiming process is performed according to each of the patterns. This method has the advantage that even for a used semiconductor wafer having unknown history, the types of films formed on the wafer can easily be examined, and a film removing method suitable for each film can be used. However, this method comprises no evaluation of the internal quality of the wafer after removal of the films, and is thus difficult to obtain an excellent reclaimed wafer when crystal defects are present in the wafer.
On the other hand, for new wafers, particularly wafers produced by the Czochralski process which occupy most of the wafers currently used for semiconductors, an analytical method is carried out by using infrared absorption spectra for controlling the oxygen concentrations of wafers in a predetermined range. This is because supersaturated oxygen present in silicon is precipitated by heat treatment performed in a device process to cause the occurrence of fine crystal defects in a wafer. This method of measuring the oxygen concentration of a wafer using an infrared spectrum is defined in ASTM F1188-88 and ASTM F1189-88, and is generally used as a standard measurement method. For example, Japanese Unexamined Patent Publication No. 6-3268 discloses a method of measuring the concentrations of interstitial oxygen and substitutional carbon in a silicon wafer by infrared absorption spectroscopy; Japanese Unexamined Patent Publication No. 7-49305 discloses a method of rapidly and easily measuring a planar distribution of inter-stitial oxygen concentrations in a silicon single crystal by using infrared absorption of a silicon wafer which appears at 1107 cmxe2x88x921. Furthermore, in a process for producing semiconductor chips, an infrared analytical method is also used for evaluating an oxygen concentration and oxygen precipitates of a wafer.
In this way, the method of measuring an oxygen concentration of a new silicon wafer or effectively removing films formed on a used silicon wafer by using an infrared absorption spectrum is carried out. However, particularly for a used wafer, a method of evaluating the internal quality of a wafer after removal of films has not been proposed yet. However, in some used wafers, defects are induced in a wafer due to film deposition and heat treatment processes, or the crystal itself deteriorates to cause crystal defects, and thus the surface quality of the final product does not reach the required level due to these defects. In recent years, particularly, the surface quality level required for the reclaimed wafer has increased with increases in the degree of integration of a device, and thus the allowable number and size of crystal defects have been strictly limited, further increasing the above-described tendency. As a result, in present circumstances, not only a decrease in the inspection pass rate due to defects or abnormality in the reclaiming process, but also a decrease in the inspection pass rate based on the deterioration in internal quality of a wafer itself cannot be neglected.
The present invention has been achieved in consideration of the above-described situation. A first object of the present invention is to provide a method of evaluating the quality of a silicon wafer which is capable of precisely detecting internal crystal defects of a silicon wafer in a nondestructive manner.
A second object of the present invention is to provide a method of reclaiming a used silicon wafer which is capable of efficiently obtaining a high-quality reclaimed silicon wafer having no internal crystal defect.
In order to achieve the first object, a method of evaluating a silicon wafer of the present invention comprises analyzing a silicon wafer by an infrared absorption spectrum, and evaluating the quality of the silicon crystal based on an absorbance ratio represented by the following formula (1):
{(Absorbance xcex11 at an arbitrary wavenumber between 1055 and 1080 cmxe2x88x921)xe2x88x92(Absorbance xcex1BL of base line)}/{(Absorbance xcex12 at an arbitrary wavenumber between 1100 and 1120 cmxe2x88x921)xe2x88x92(Absorbance xcex1BL of base line)}xe2x80x83xe2x80x83(1)
In the formula, absorbances xcex11 and xcex12 represent absorbances of the measured silicon wafer, and base line absorbance xcex1BL represents the absorbance of a base line of the measured silicon wafer, which is drawn from 1030 to 1170 cmxe2x88x921.
In order to achieve the second object, a method of reclaiming a used silicon wafer of the present invention comprises the steps of removing films formed on a silicon wafer, and then evaluating the quality of the silicon crystal of the silicon wafer based on the above method.